This invention relates generally to apparatus for cooling packages containing pre-cooked food products, and more particularly to a freezer unit for use in a cold chamber and adapted to contain a load of such packages with air spaces therebetween, the unit functioning to draw cold air from the atmosphere of the chamber and to circulate this air within the unit to cause it to flow at high velocity in the air spaces to promote rapid cooling of the food products.
The above-identified related patent applications disclose a convenience food technique and apparatus therefor in which pre-cooked meals which have been kept under refrigeration are thereafter reheated in a hot air oven and made directly available to customers. The nature of the cooking, refrigeration and reheating operations are such that the essential texture, flavor and nutritional value of the food are maintained without significant degradation.
In this convenience food technique, each cooked meal is placed in an individual tray and sealed, a stack of such trays being nested within an open carton to form a cartridge which facilitates subsequent handling and processing. The side walls of the carton are provided with holes to admit heated or low temperature air which is circulated in the spaces between the trays in the stack to heat or cool the meals therein, as the case may be.
These cartridges, prior to being placed in an oven for reheating the pre-cooked meals, must be kept in a refrigerated storage chamber to maintain the food temperature just above the freezing point for as long as is necessary. In practice, this temperature may be in the range of about 12.degree. to 30.degree. F.; for when the moisture content of the food is rich in dissolved salts, the freezing point may be well below 32.degree. F. It is important that the trays be sealed to prevent the loss of moisture and volatile constituents.
In the large scale production of cooked meals in connection with a convenience food technique of the type disclosed in the related patent applications, after the meals have been cooked and placed in sealed trays, it is essential that the temperature of the hot meals in the trays be reduced in temperature quickly to a level approaching the cold temperature of the refrigerated chamber in which the trays are to be stored. To this end, a blast tunnel may be used, but this is costly to operate, particularly for small production runs.
In a prior attempt to accelerate the action of a conventional freezing system and to overcome certain shortcomings thereof, the Overbye U.S. Pat. No. 3,115,756 discloses an arrangement in which the food to be cooled or frozen is conveyed through an open tunnel on a foraminous belt below which are disposed the evaporation coils of a mechanical refrigeration system. Air is blown upwardly through the coils and the resultant cold air is then forced through the belt to effect rapid cooling of the food advancing through the tunnel.
While the Overbye arrangement is more efficient than most blast tunnel freezers, it has serious drawbacks, among which is the formation of snow and ice on the refrigeration coils and other components of the system as a result of moisture extracted from the food being processed and from the ambient air. The formation of such snow and ice markedly reduces the thermal efficiency of the refrigeration system.
Thus in a convenience food operation, various meals to be later served on demand are first cooked and then deep-frozen and stored. When an order is placed for a particular meal, the selected item is withdrawn from the freezer and then thawed and reheated. A particular concern of the present invention are convenience food operations carried out in institutional, cafeteria and other mass-feeding facilities wherein various entrees are pre-packed and pre-cooked in steamable trays. Typical convenience food products for this purpose are frozen pasta entrees prepared by Buitoni Food Service of Hackensack, N.J. One such entree takes the form of beef-stuffed pasta shells, twenty such frozen shells being contained in a single steamable tray.
The term "steamable tray" refers to a flanged tray of rectangular form made of aluminum foil and having a removable foil cover to seal the tray. Such trays come in standard sizes but in different depths for use in conjunction with steam tables or bains-marie, these being tables having openings therein to receive and hold trays of cooked food over steam or hot water circulating beneath them.
The term "pre-cooked food package" as used herein encompasses the sealed trays of the type disclosed in the above-identified related patent applications in which the tray also functions as the serving plate, the sealed "steamable" trays in commercial use in which the food must be transferred to plates, and any other form of sealed tray or pan containing a pre-cooked meal or food product.
It is essential when freezing a meal that has just been cooked that freezing take place within a relatively brief period; for the longer it takes to effect the necessary freezing, the greater is the danger of deleterious bacterial activity that might spoil the product. Thus, assuming the availability of a convenional walk-in freezer closet or cold chamber in a cafeteria or institutional mass-feeding institution, should a load of pre-cooked food packages being placed in the chamber whose atmosphere is cold air, in the typical chamber it will ordinarily take 12 to 14 hours before the load is frozen.
A prolonged cool-down period is not only unacceptable in terms of bacterial activity, but the long term presence of a load of pre-cooked food packages in the chamber pre-empts a considerable space in the chamber which is then not available for storing other food products requiring refrigeration, such as meats and vegetables.
Moreover, a protracted cool-down period can also be destructive of the texture of the pre-cooked food products. With freezing, the water or moisture content of the food is converted into ice crystals, the size of the crystals being a function of freezing time. When freezing occurs gradually, large ice crystals are formed which act to rupture the internal structure of the food product, and in some cases to render it mushy and unpalatable. Fast-freezing, on the other hand, gives rise to much smaller ice crystals with minimal destructive effects.
With existing freezing techniques, it is not possible in the context of a typical walk-in cold chamber or similar refrigeration facility to effect rapid and non-destructive freezing of pre-cooked food packages. The reason for this drawback is attributable to two heat transfer factors, flow velocity and film coefficient.
The process by which a package of pre-cooked food at a temperature above ambient is lowered in temperature to a freezing level involves the transfer of heat from a point at a high temperature to a point of low temperature. Methods of heat transfer include conductive heat flow through solids and liquids, and convection currents in fluids.
In thermal convection, heat is transferred by the movement of fluid matter; whereas in conduction, atoms or molecules simply pass on the kinetic energy delivered to them. In either case, the rate of heat transfer depends on the temperature gradient and the cross-sectional area of the heat transfer path.
When heat is carried away by forced convection from a warm food package by a stream of cold air, the greater the air velocity, the more rapid the cooling process. The static cold air in the typical cold chamber does not provide a high velocity stream which promotes freezing. Moreover, a conduction factor also comes into play in the cooling process, for a thin layer of fluid remains stagnant against the interface of the cold air stream and the warm body being cooled. This film, in effect, becomes another conducting layer outside of which the fluid stream temperature exists.
Though this stagnant film is extremely thin, its resistance to heat transfer is usually high compared to heat flow in a solid body. Thus a heat exchanger in which the solid material is a highly conductive metal, the film at the interface introduces significant heat flow resistance between the metal body and the fluid in heat exchange relation therewith. When, however, the body of food being frozen is in a sealed package which is placed in a cold air stream that flows past the surface of the package at high velocity, this tends to dislodge any film at the external surface of the package. However, as will hereinafter be explained in greater detail, internal films are formed in the packaged body of food which militate against rapid heat transfer. These internal films slow down the freezing process and result in the formation of large ice crystals.